1. Field of the Invention
The present invention relates to a thin film transistor and a manufacturing method thereof, and more specifically, to a thin film transistor which is formed, for example, on a substrate of a display device and the like, and a manufacturing method thereof.
2. Description of the Related Art
In recent years, with the expansion of the market for cellular phones or digital single-lens reflex cameras, there has been a vigorous development in high-definition liquid crystal displays (liquid crystal display devices) or organic EL displays (organic EL display devices), and the like. A big problem in the development of these displays lies in the difficulty of achieving both high performance and low cost in thin film transistors for driving a so-called backplane.
However, although conventional amorphous silicon (a-Si) thin film transistors can be formed at low cost, they have a problem in that the field-effect mobility thereof is as low as about 0.5 cm2/Vs and the threshold voltage thereof is shifted by an amount of 1 V or more within 100 hours. Moreover, although conventional low-temperature polycrystalline silicon (LTPS) thin film transistors have high performance, they have a problem in that the manufacturing method thereof involves laser annealing which increases the number of manufacturing processes and thus cost-reduction is difficult. For these reasons, in recent years, lots of attention is given to so-called microcrystalline silicon thin film transistors in which a higher field-effect mobility and a lower threshold voltage shift than the a-Si thin film transistors can be expected and which can be manufactured at lower cost than the LTPS thin film transistors.
As a method of forming the microcrystalline silicon, for example, a method that uses a thermal annealing technique, a laser annealing technique, and a plasma enhanced chemical vapor deposition (CVD) method is known. However, the annealing technique has a problem in that it involves a number of manufacturing processes similarly to LTPS thin film transistors. Moreover, the plasma enhanced CVD method has a problem in that since an incubation layer formed in the initial stage of deposition has poor crystallinity, it is difficult to achieve an improvement in field-effect mobility and a reduction of the threshold voltage shift in thin film transistors using microcrystalline silicon formed by the plasma enhanced CVD method. As a substitute for these methods, a reactive thermal chemical vapor deposition (CVD) method which can form crystalline nuclei directly on an insulating film at a temperature of about 500° C. by using a redox reaction of source gases is anticipated to be an effective microcrystalline silicon deposition technique. When this technique is put into practical use, since it is compatible with the a-Si process, it may be possible to form thin film transistors with few manufacturing processes. Further, through the use of high-quality microcrystalline silicon, it may be possible to achieve an improvement in field-effect mobility and a reduction of the threshold voltage shift in the thin film transistors.
JP 2007-165921 A is an example of the related art of the present invention. According to the invention disclosed in JP 2007-165921 A, after crystalline nuclei are formed on a substrate by a thermal chemical vapor deposition (CVD) method, crystals are grown using the crystalline nuclei with a low-temperature crystal growth technique. In this way, a polycrystalline film which has high crystallinity and of which the crystal grain size and the crystal orientation are controlled appropriately can be formed with ease at a low temperature.